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I am currently planning on building some grow-houses/cold frames. Actually mostly raised beds 18" (45cm) high, with the ability to fit panels over it in winter turning them into cold frames.

The rear wall will be north facing and against the neighbours 6 foot fence. This means that the sunlight will never come from this direction.

I will be using twinwall polycarbonate for the window panels. I have thought that the rear panel may be permanent and use a thicker, and therefore more insulating panel for this. The rear panel being around 30% of the surface and I can reduce heat loss through this section without effecting the light reaching plants.

Would it be better to leave the rear panel clear, make it white to aid light reflecting on to plants or make it black to aid heat capture?

I am in East Lothian, Scotland. During summer it will receive 11 hours of direct sunlight, during Autumn around 4 hours. In Winter there will likely be no direct sunlight but it will be light for 8 hours.

My intention is to extend the growing season, maximising crop production as well as protecting from frost.

Update

I mentioned raised beds which could have panels over the top, I was referring to some thing like this, to work with the square foot/meter garden. An alternative mentioned by @bamboo which I had not thought of would be 2 panels leaning on each other to make a cloche type arrangement.

Morgan
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Cold frames and cloche type arrangements are not the same thing. I'd leave the cloche ones clear, probably make the whole thing out of twin polycarbonate. It has insulating properties of its own anyway, and although you think no light will get to the plants because of the fence, that isn't always true. For instance, if the fence were white, or a pale colour, then light will be reflected. Cloches and the like are always clear sided as well as clear on the top, thus maximising any available light, ambient or direct. Cold frames, on the other hand, usually just have a clear top, but the local horticultural college here has some which are clear all round, apart from where they join the wall at the back. Cold frames are usually fixed, meaning that they're not moved around, usually being too unwieldy for that.

Other thoughts on extending the growing season (imported from Chat with Organic Lawn)

You could extend your growing season simply by pegging thick black plastic sheeting onto the soil early in the year, then remove it to plant in at least 4 weeks earlier than you'd normally be able to. I've discovered you can buy water filled cloches (who knew!) and they look interesting - they don't need ventilating. The water increases the R-Value and helps retain heat.

I don't know how much space you have, but if you have a couple of very good, contained compost heaps which get hot, I once saw a guy who made himself movable 'beds' about 9 inches deep, out of wood, filled with soil, which he'd rest over wood supports on the compost heap, and grow things out of season (lettuce etc) which thrived on the heat from the heap beneath, despite the cold air temps.

Bamboo
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One thing you left out of your question is where the heat is coming from. Understanding this might help you come up with a better design. Please bear with me.

A cold frame extends the growing season by keeping the local environment within the cold frame more comfortable for the plants than the colder outside environment.

To retain heat, there has to be a source of heat. In a cold frame, the source of heat is going to be the little bit of heat generated by the plants themselves and the heat stored in the air and soil. Air escapes easily but the soil stays put and has a high thermal mass.

During the day, the transparent areas allow light in, which the plants need. The light also heats up air and soil in the cold frame. The soil will also have heat stored from warmer seasons.

At night, when it gets colder, the cold frame's insulation slows the transfer of heat from the inside to the outside. Heat is also lost through the bottom of the bed as well as the sides. More from the sides than the bottom. The taller you make the sides of your bed the more easily heat will transfer between the soil in the beds and the air surrounding them. The soil in taller beds will heat up faster in the spring but also cool down faster in the winter.

As Bamboo indicated in the answer to your previous question, when you go with higher insulated twinwall, you reduce the amount of light that comes in so you need to get the right balance.

Your cold frame cover will have 5 sides (1 top, 4 sides). To maximize heat retention make as many of those sides out of 2x lumber as you can. Wood has a higher thermal mass than air and plastic (twinwall). It will store heat during the day and release it at night.

To let the most amount of light in, make the sides that receive the most light out of twinwall.

So, yes, don't use clear twinwall for sides that won't receive light but use wood instead of plastic to increase insulation and thermal mass.

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OrganicLawnDIY
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  • I'm not clear, so thought I'd ask for clarification - your comments are referring to cold frames rather than moveable cloche type arrangements? – Bamboo Sep 13 '13 at 14:30
  • I'm referring to cold frames that are added on top of raised beds which is what I understood Morgan to be asking. – OrganicLawnDIY Sep 13 '13 at 14:46
  • Morgan didn't state this in the question but I also know he is planning on doing Square Meter Gardening based on a conversation we had on Twitter so I assumed he's looking to do something like this http://gardenlogblog.wordpress.com/2010/04/26/diy-raised-bed-cold-frames/ – OrganicLawnDIY Sep 13 '13 at 14:58
  • Maybe I misunderstood the question - the term cold frame means something specific, and it ain't moveable, but also he says 'mostly' on raised beds, so I assumed he was talking about cloche type affairs AND separate cold frames. – Bamboo Sep 13 '13 at 17:08
  • @Bamboo Us amateurs sometimes mix up the terminology which might cause confusion to someone like you who knows better. :) To me, the first line reads like he's looking to build some kind of cold weather cover to place on top of a raised bed, turning the raised bed into a sort of cold frame. Hopefully it gets cleared up. I always thought a cloche was a small protector for one or 2 plants but recently read row covers are also called cloches. The pic in my previous comment should probably be called a cloche? Permanent box, clear top, wooden sides is a cold frame? – OrganicLawnDIY Sep 13 '13 at 18:24
  • Yep - cold frame means something with a clear top that's usually quite large, though not always, and isn't moved around. Cloche doesn't exactly describe what Morgan wants to make to use on open ground to warm it up, because its going to be bigger, but it certainly wouldn't be called a cold frame! I'm going to check out if there's a suitable substitute for the word cloche in these circs. – Bamboo Sep 14 '13 at 11:45
  • let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/10599/discussion-between-organiclawndiy-and-bamboo) – OrganicLawnDIY Sep 14 '13 at 12:47